In an image forming apparatus of an electrophotographic system (including an electrostatic recording system), such as a copying machine, laser beam printer or facsimile, a developer is used for making an electrostatic latent image formed on a photosensitive member visible. The developer comprises, as a main component, colored particles (toner) with a colorant, a charge control agent, a parting agent and the like dispersed in a binder resin.
The toner is roughly divided into a pulverized toner obtained by a pulverization process and a polymerized toner obtained by a polymerization process. In the pulverization process, a pulverized toner is obtained as colored resin powder by a process, in which a thermoplastic resin is melted and kneaded together with additive components such as a colorant, a charge control agent and a parting agent, and the resultant kneaded product is pulverized and classified. The thermoplastic resin used in the pulverization process is synthesized by polymerizing a polymerizable monomer in advance. On the other hand, in the polymerization process, a polymerized toner is obtained as colored polymer particles by a process, in which a polymerizable monomer composition containing a polymerizable monomer and additive components is polymerized in the presence of a polymerization initiator in an aqueous dispersion medium.
Even in any process, it is difficult to completely polymerize the polymerizable monomer in a polymerization step, and it is not avoided that an unreacted polymerizable monomer remains in the toner. The unreacted polymerizable monomer (hereinafter also referred to as “the remaining monomer”) remaining in the toner causes such various problems that (1) the polymerizable monomer is vaporized out of the toner by heating upon fixing, or the like to worsen a working environment or emit offensive odor, (2) the toner undergoes blocking during its storage, (3) the flowability of the toner is deteriorated to lower the quality of an image formed with such a toner, (4) the toner is easy to cause offset, and (5) the toner is easy to cause toner filming on individual members in an image forming apparatus even though the amount thereof is slight.
The problems caused by the remaining monomer are more serious in the polymerized toner than the pulverized toner. In case of the pulverized toner, the content of the remaining monomer easily reduced by a heat treatment and a drying treatment in a preparation stage and a melting and kneading stage of the thermoplastic resin which will becomes a binder resin. On the other hand, in case of the polymerized toner, the remaining monomer must be removed from the polymer particles containing the additive components such as the colorant, charge control agent and parting agent. However, the remaining monomer is easy to be absorbed in these additive components, so that it is difficult to reduce the content of the remaining monomer compared with the case of the binder resin alone. In addition, the polymerized toner is easy to aggregate or fuse, so that there is a limit to the demonomer treatment by the heat treatment or the like. In recent years, there has been a strong demand for development of a polymerized toner capable of being fixed at a low temperature for the purpose of achieving speeding-up of printing and formation of full-color images. However, it is extremely difficult in such a low-temperature fixing toner to reduce the content of the remaining monomer while preventing aggregation or fusion.
The polymerized toner contains various volatile organic compounds secondarily produced in the polymerization reaction in addition to the remaining monomer. The volatile organic compounds are vaporized out of the toner by heating upon fixing, or the like, so that the volatile organic compounds have an important adverse influence on the toner properties like the remaining monomer. In particular, when a polymerization initiator high in initiation efficiency is used, volatile organic compounds such as ether compounds are easy to be formed by side reactions, and such compounds come to be contained in the resulting polymerized toner. The unreacted polymerizable monomer and the other volatile organic compounds will hereinafter be referred to as volatile organic components (VOC) collectively.
As a method for reducing the content of the remaining monomer or volatile organic compounds in the polymerized toner, it has heretofore been known to polymerize a polymerizable monomer composition comprising a polymerizable monomer and a colorant in an aqueous dispersion medium and then subjecting a dispersion containing polymer particles formed to a stripping treatment.
As a stripping treatment method using saturated steam, for example, Japanese Patent Application Laid-Open No. 5-100485 (Patent Literature 1) discloses a stripping treatment method, in which after a polymerizable monomer composition comprising a polymerizable monomer and a colorant is suspension-polymerized, a suspension (dispersion) containing toner particles (polymer particles) is heated, and water is then distilled out of the suspension while introducing saturated steam of 100° C. into the suspension. According to the stripping treatment method of Patent Literature 1, the content of the remaining monomer can be reduced to 90 ppm (Example 1).
Japanese Patent Application Laid-Open No. 2004-4383 (Patent Literature 2) discloses a stripping treatment method, in which after a polymerizable monomer composition comprising a polymerizable monomer and a colorant is suspension-polymerized, saturated steam higher than 100° C. is introduced into an aqueous medium (dispersion) containing toner particles. The stripping treatment method of Patent Literature 2 has a feature in that saturated steam of a high temperature exceeding 100° C. is continuously blown into the dispersion containing polymer particles to increase the amount of the aqueous medium while retaining the temperature of the aqueous medium at the boiling point (100° C.).
Japanese Patent Application Laid-Open No. 2006-208624 (Patent Literature 3) discloses a stripping treatment method, in which a carrier gas is introduced into a polymer dispersion containing toner particles to remove organic volatile components (volatile organic components). In Patent Literature 3, saturated steam is shown as the carrier gas.
However, the stripping treatment method using saturated steam of a high temperature by itself takes a long time to reduce the content of the remaining monomer or volatile organic components, and the degree of reduction is not always sufficient, so that this method is not efficient. In addition, when the high temperature saturated steam is blown into the dispersion containing the polymer particles, the polymer particles are easy to cause partial aggregation or fusion. In particular, when the method of continuously blowing the saturated steam of the high temperature exceeding 100° C. is adopted, the temperature of the dispersion is raised even to 100° C. during the stripping, so that the polymer particles designed to be a low-temperature fixing type are easy to fuse or change the surface properties thereof. Further, according to the method of blowing the saturated steam of the high temperature, the liquid level of the dispersion is greatly raised during the long-time stripping treatment, or it is difficult to control the level of a bubble layer on the liquid level. When water is distilled off during the stripping treatment for inhibiting the rise of the liquid level, the quantity of heat for distillation is required, and the process thereof becomes complicated.
On the other hand, as a stripping treatment method using an inert gas, Patent Literature 3 also discloses a method using, as the carrier gas, the inert gas in addition to the saturated steam.
Japanese Patent Application Laid-Open No. 2004-271816 (Patent Literature 4) discloses a method, in which in a step of subjecting a dispersion containing polymer particles to a stripping treatment to remove the remaining monomer in the polymer particles, a gas such as an inert gas is blown while controlling a bubble level on the liquid level of the dispersion.
Japanese Patent Application Laid-Open No. 2007-65426 (Patent Literature 5) discloses a stripping treatment method, in which a gas composed of air or an inert gas is blown into a dispersion containing colored polymer particles (polymerized toner) in an evaporator, and at this time a flow rate of the gas and the pressure of a vapor phase in the evaporator are controlled within respective specified ranges.
According to the stripping treatment method of blowing the gas such as the inert gas into the dispersion containing the polymer particles, the content of the remaining monomer can be reduced while preventing the aggregation or fusion of the polymer particles. In addition, the flow rate of the inert gas is controlled, whereby the level of a bubble layer generated on the liquid level of the dispersion can be controlled.
However, the conventional stripping treatment method using the saturated steam or inert gas by itself is not sufficient to efficiently and sufficiently reduce the content (TVOC content) of total volatile organic components including the remaining monomer.
According to the results of investigation by the present inventor, it has been proved that the stripping treatment method of blowing the inert gas such as nitrogen gas into the dispersion containing the polymer particles is insufficient in the efficiency to remove the volatile organic compounds, and it is thus difficult to reduce the TVOC content in the resulting polymerized toner to a sufficient and low level. In addition, this polymerized toner is not sufficient in printing durability. In particular, when a printing durability test is carried out on this polymerized toner under a high-temperature and high-humidity environment, it is difficult to sufficiently increase the number of paper sheets, on which printing can be continuously conducted up to occurrence of fog. It has more importantly been proved that vertical stripes are easy to occur on printing paper in a stage that the number of paper sheets continuously printed is relatively small even when the printing durability test is carried out on this polymerized toner under a normal-temperature and normal-humidity environment.
The stripping treatment method using the saturated steam involves such problems as described above, the TVOC content in the polymerized toner obtained thereby is at a high level, and the printing durability test on this polymerized toner results in the fact that the number of paper sheets continuously printed is smaller compared with the polymerized toner obtained by conducting the stripping treatment with the inert gas.
In recent years, there has been a tendency for environmental regulations to be tightened, and so the image forming apparatus of the electrophotographic system have also been strongly required to more reduce the volatile organic components volatilized upon fixing of a toner under heat, or the like.    Patent Literature 1: Japanese Patent Application Laid-Open No. 5-100485 (corresponding to U.S. Pat. No. 5,476,745)    Patent Literature 2: Japanese Patent Application Laid-Open No. 2004-4383 (corresponding to US 2003/0224276 A1)    Patent Literature 3: Japanese Patent Application Laid-Open No. 2006-208624    Patent Literature 4: Japanese Patent Application Laid-Open No. 2004-271816    Patent Literature 5: Japanese Patent Application Laid-Open No. 2007-65426 (corresponding to US 2007/0048645 A1)